Of all the tricky things to paint, human flesh is the most difficult to represent accurately.

Historical distribution of skin tones according to levels of UV radiation.

It's not that we're such a rainbow, from from it; we're not wildflowers or birds of paradise. In human coloration, there's only a few elements in play, a limited gradation from pitch brown-black to transparent, pink-tinged ivory: the brown-black eumelanin, the red-brown pheomelanin, and beneath that filter, the red of our blood. Only a creature as self-regarding and egomaniacal as we are can mine political hierarchies out of these shades of brown, but be that as it may, our eyes are extraordinarily sensitive to the minutiae of the human face. A lipstick that's slightly off, we're not conscious why the woman doesn't look pulled together, until she swaps it for one that flatters and instantly it clicks: aha, so that's what was missing.

If the purpose of makeup is to create illusions, then choosing the right colors is all about creating optical illusions, shifting natural coloring into optimum balance. Consider J. Lo, lately become a parody of herself. In all likelihood, she was younger when photographed in that red lipstick, but there's a discernible glow added to her skin and eyes, once she adopts her signature nude.

Browns are complex—mixtures of other, more primary colors—they absorb rather than reflect light. The purpose of melanin is to protect us from the damaging effects of the sun. This is why, much to the exasperation of black women everywhere, pigments disappear on darker complexions; the melanin performs exactly as nature intended, swallowing up light. In latitudes where melanin hinders absorption of vitamin D, its dilution, like a drop of oil refracted on rain-slick asphalt, reveals its true colors. At saturation, eumelanin is responsible for black hair, but just a drop stretched out over a strand manifests as blond. Physiological differentiation may have yielded vastly different phenotypes, but pale Asians share more shades in common with cool blondes than with more olive-yellow hued (the presence of pheomelanin) women within the same race.

At present, there is no satisfactory system for categorizing skin tones, at least one comprehensive enough to accommodate a wide range of ethnicities. The alternative, matching foundations to your skin, only serves as reference; whether you're a MAC C25 or a Kanebo YO02, you gain no further direction as to which colors will flatter you best. Consequently, let's start from the ground up. We may not end up with a reliable system of prediction—you'll still have to experiment with colors (that's half the fun)—but at least it'll cut away some of the confusion.

DEPTHThe most basic and readily obvious characteristic: lighter skin tones are dilute in melanin, darker complexions are saturated. When exposed to intense sunlight, some people tan (melanogenesis), others simply burn. While depth of skin does not indicate specific color families, as a general rule, the deeper your skin tone, the richer the pigmentation required.

UNDERTONESIf depth is the concentration of melanin in the skin, undertones are determined by the proportions of the pigmentation. To my eye, human skin tones are mixtures of three basic undertones: yellow, olive, and pink.

The most common undertone, yellow, is caused by the presence of eumelanin, the brown-black pigment, which dilutes to a golden tint. On pale complexions where yellow is dominant, the underlying blood lends the skin a peachy tone. In many cases, yellow undertones are mixed with lesser quantities of olive, the proportions individual to every person. Consequently, warm skin tones exhibit the greatest variation, especially at medium strength; many darker Asian, hispanic, and light-skinned black women can be said to have golden undertones, but the colorings are quite dissimilar. Yellow undertones are frequently described as warm (except at MAC), primarily because warm blushes like peach and coral provide the ideal complement, while a cool, blue-leaning pink is jarring. If you lean only slightly yellow, then a peach-pink blush hits the sweet spot; no surprise that this is the most popular shade of them all. Often, women with strong yellow undertones, which carries a negative connotation in many cultures, bleach their skin, or lighten their hair, as the dilution creates the optical illusion of a more balanced complexion, or lavish their skin with fake tanners and bronzers, as bronze is richer in tone.

We're guessing that olive undertones are primiarly the result of pheomelanin, the pigment responsible for red hair in the absence of eumelanin, since many natural redheads also have an olive tinge to their skin. Olive undertones occur less frequently than yellow; many cultures originating in the Mediterranean (Jewish, Italian, Arab, Spanish, etc.) do feature a strong tendency towards olive, but it's not particularly useful to approach the question of undertones from an ethnic perspective, as many women with East-Asian and Celtic ancestry often have olive skin. Strong olive undertones, especially those with lighter complexions, can make the face look fatigued, even ill, what's often described as "sallow", best remedied by mauve or plum blush, properly muted, or a dusting of bronzer. If the skin is pale, a lavender-tinged powder cancels out the green for a porcelain effect; at medium strength, olive skin beige to tan, and quite flat, so metallics are recommended. Olive skin is not receptive to bright colors, especially pastels, and women often find it's easy to overdose on makeup. Jewel tones, such as forest green on the eyes or a berry lipstick, can be extremely wearable, but on the whole, the most reliable shades are complex neutrals and earth tones. They may be muddy in the pan, but somehow they come alive over olive undertones. For that reason, olive skin is frequently described as "neutral", a pretty meaningless label.

Everyone's skin has some pink, from the blood beneath the surface of the skin, but in most cases, melanin shifts the color in other direction, like a filter. In that sense, we might consider pink the absence of undertones, skin transparent enough that the dominant color is not the skin itself. Pink is the least common of the three undertones. It should not be confused with surface ruddiness, from acne or broken capillaries or rosacea, but should be an allover pink from beneath the skin. Though darker complexions may take on a pink hue, provided the skin is quite thin and clear, it more frequently occurs on pale skin, where melanin is dilute to begin with (albinism is the extreme). For that reason, pink undertones seem lighter than yellow or olive undertones; Asians with pink undertones are often described as "white-skinned", though of course they are not that pale. With fewer competing undertones, this "cool" complexion, according the usual parlance, can accommodate a greater diversity and intensity of colors, though icy blues and bright pinks seem to be preferred. Indeed, without color, pink complexions look washed out and drained of life, unless of course you flush easily. On the other hand, brown-based neutrals, which replicate skin tones, look like a smear of dirt; opt for greys and navy, instead.

Somewhere in between the extremely olive Natalie Portman and the decidedly pink Christina Hendricks, lies pink/olive Angelina Jolie, with emphasis on the olive. While some people have a single dominant undertone, the vast majority are mixed, which is why categorizing skin tones is so complicated.

It takes an experienced eye to pick out undertones. If you're interested in figuring yours out, examine your complexion while wearing a white shirt, which emphasizes undertones (black diminishes). The surest litmus test is blush, since it is the cosmetic that works most in concert with natural skin tones. It is not fail safe, as hair and eye color can shift things—Hendricks naturally a blonde, must wear warmer, peachy-toned blush to keep her pink undertones from clashing with her ersatz red—but between a vivid orange blush (flatters yellow undertones), a pure pink (picks up on pink), and a rich plum (complements olive skin), one is bound to blend out into a convincingly natural glow. Make sure to cancel ruddiness with foundation first, if necessary. Cream blushes are easier to overapply then sheer out, but powder blushes, even lipsticks, will do as long as the color is intense, even harsh, and as you're grimacing at the grotesque pink cheeks in the mirror, you can be certain your undertones are not pink-dominant, i.e. "cool". If you're shy about approaching the counter, then experiment with three blushes from the drugstore at home.

reference colors for a general sense of direction, not necessarily the ultimate shade for you

Few people can carry off a shade as vibrant as Stila Gladiola (shown above), but if it tests better than Bobbi Brown Pale Pink or Becca Lotus, you can branch out into less aggressive members within the same color family: peach, apricot, coral, terracotta, brick red, red-chocolate, etc. Gladiola has served its purpose: a little red mixed in with yellow, just like your skin.

DISCOLORATIONIt's not exactly necessary to know your undertones. We gravitate towards colors that flatter, by instinct rather than design, not knowing they flatter because they complement our undertones. Nor do we always want what's best for us; sometimes, a contrast is more fun than a complement. But as our eyes are so sensitive to subtle shifts in color—when rival companies shamelessly reproduce a bestselling shade, reviewers are quick to spot the differences—a little education does facilitate the process of refinement. There are some with no dominant undertone, truly neutral, with pink, yellow, and olive in harmonious balance. She can pull off any color. A great example is Natalia Vodianova, among many models, who handles the entire rainbow with ease. The rest of us, more disproportionate, must make slight adjustments and choose colors in better concert with our undertone(s), shades that shift the balance back towards neutral.

In extreme cases, it is possible to correct strong undertones. If you're more adventurous, you can correct sallowness with lavender, ruddiness with mint green, but both must be blended beneath normal skin tones.

Almost everyone has some localized discoloration beneath the eyes from the minute blue veins beneath the surface of the skin: usually a mauve-tinged brown, sometimes greenish-brown. Undereye concealer that leans slightly pink, peach, salmon, even mauve, depending on the depth of your skin, brightens better than one that matches your skin exactly. It's the principle behind the infamous Touche Eclat (by Terry de Gunzberg), a milky pink-peach highlighter; instead of covering discoloration, it corrects the purple or green tones. To lift sallowness and fatigue allover, a tinted loose powder has just enough transparency for subtle brightening: a cool pink like T. LeClerc Orchidee or the soft peach of Chanel Poudre Universelle in Clair. Since shimmers draw light, a highlighter with a dimensional shimmer—silver, white, pink, lavender, mint, blue, gold, peach—scatters it better than one solid color, though the proportions should be shifted to your undertones. Silvery pinks work better on cooler complexions, peachy golds on warmer: to test, try on a silver-based shadow on one eye, makeup-free, gold-based on the other—one will brighten better, even without the aid of concealer.

For ruddiness, a similar principle applies, only worked out in yellow. If the irritation is more generalized, from broken capillaries or irritation, then a yellow-toned powder, like Bobbi Brown's, might suffice. Primers are also an option, and you can risk a stronger yellow or mint if you've got serious rosacea, since foundation will soften the impact. For blemishes, while the depth should match your skin tone exactly, a hint of yellow to the flesh tone never hurts, and it is not too difficult to source considering that yellow-based bases are now the norm; it helps to cancel the red better.